The mammalian neocortex is an enormous network of cells, each making thousands of connections and an array of neurological conditions can result from inappropriate cortical structure or connectivity. Many of these congenital brain defects have a genetic origin but we still lack a full understanding of the genes and mechanisms involved. The overall objective of this application is to use forward genetic approaches in mouse and human to identify and validate novel alleles important for development of cortical circuitry and overall structure. Our central hypothesis is that a synergistic and unbiased forward genetic approach in mouse and human will lead to fundamental discoveries in the genetics of cortical circuit formation and structural development. The rationale of this proposed research is that by identifying novel genes through forward genetic approaches which are required for normal cortical development using both human and mouse genetics, we are then positioned to use this information and tools to study the etiological mechanisms of human cortical malformations in subsequent studies. We will test this central hypothesis and accomplish the goals of this application by pursuing the following three specific aims: 1) use forward genetics in the mouse to efficiently generate and capture genetic mutations in loci important for cortical circuit formation and structural development, 2) identify and validate causal mutations in novel mouse models of cortical circuit formation and structural brain defects, and 3) apply next-generation sequencing approaches to identify mutations leading to human movement disorders and structural brain defects. The aims are accomplished by an ENU mutagenesis approach in the mouse with the addition of a novel transgenic reporter which is expressed specifically in cortical layer V pyramidal neurons. The mutations are then cloned and validated through a number of functional studies. The human genetics studies are performed with the application of exome sequencing to carefully selected familial cases of movement disorders and structural brain malformations. These studies will identify several genes essential for mammalian forebrain structure and function. The significance of this work is found in the specific application to cortial circuitry and structure, and that an unbiased approach such as this has the capability to implicate entirely new pathways in neurological disease. A synergistic approach using both mouse and human genetics to specifically query these aspects of neural development allows fundamental insights into the genetics of development and disease. Such knowledge is not only critical to further understand the basic mechanisms of neurodevelopment, but also has immediate clinical relevance through identification of a number of potential therapeutic targets. Furthermore, these mouse models provide a reusable resource to directly characterize the role of the mutated gene in neurodevelopment, and potentially serve as a tool to test future therapeutic interventions. Taken together, these findings are therefore applicable to basic developmental neurobiology, pediatric and adult neurology, human genetics and genetic counseling.